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Hooking into Node.js stdout
var util = require('util')
function hook_stdout(callback) {
var old_write = process.stdout.write
process.stdout.write = (function(write) {
return function(string, encoding, fd) {
write.apply(process.stdout, arguments)
callback(string, encoding, fd)
return function() {
process.stdout.write = old_write
var unhook = hook_stdout(function(string, encoding, fd) {
util.debug('stdout: ' + util.inspect(string))



Thanks, I've created a use case example for when one uses cluster and wishes to separate per-process output.

module.exports = function(mediator) {

    var fs          = require('fs'),
        path        = require('path'),
        config      = require('../../config'),
        env         = process.env,
        stdout      = process.stdout,
        stderr      = process.stderr,
        workerId    = env.NODE_WORKER_ID || env.NODE_UNIQUE_ID,
        logDir      = config.debug.logDir,
        stdoutFile  = path.resolve(logDir, 'worker#' + workerId + '.log'),
        stderrFile  = path.resolve(logDir, 'worker#' + workerId + '.err.log');

    //run prior to boot ready
    mediator.once('boot.init', function() {

        //create a new stdout file stream
        var stdoutFS = fs.createWriteStream(stdoutFile, {
            encoding: 'utf8',
            flags   : 'a+'

        //create a new stderr file stream
        var stderrFS = fs.createWriteStream(stderrFile, {
            encoding: 'utf8',
            flags   : 'a+'

        //pipe stdout to a worker file
        var unhookStdout = hook_writestream(stdout, function(string, encoding, fd) {
            stdoutFS.write(string, encoding || 'utf8');
        console.log('\n\nPrepared new stdout hook to worker file.');

        //pipe stderr to a worker file
        var unhookStderr = hook_writestream(stderr, function(string, encoding, fd) {
            stderrFS.write(string, encoding || 'utf8');
        console.log('\n\nPrepared new stderr hook to worker file.');

        //unhook when things go wrong
        stdoutFS.once('close', function() {
            console.log('Unhooked stdout.');
        stdoutFS.once('error', function(err) {
            console.error('Error: Unhooked stdout due to error %j.', err);
        stderrFS.once('close', function() {
            console.log('Unhooked stderr.');
        stderrFS.once('error', function(err) {
            console.error('Error: Unhooked stderr due to error %j.', err);


    function hook_writestream(stream, callback) {
        var old_write = stream.write;

        stream.write = (function(write) {
            return function(string, encoding, fd) {
                write.apply(stream, arguments);
                callback(string, encoding, fd);

        return function() {
            stream.write = old_write;


Thanks for this :)




Crystallized it a bit further here:


Awesome magic here. :smile: Thanks!


Thanks @pguillory, I've used it on debug-logger


Genius! In my implementation, I created a new object to add my changes to:

var stdout = process.stdout;
var shadow = process.stdout = Object.create(process.stdout);
shadow.original = stdout;
shadow.write = function() {
    // stdout.original.write.apply(stdout, arguments); // not shown
    result.stdout += data;

(Which didn't work. Ignore me. ;P)

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